Conventionally, as a type of tractor, there has been known a tractor in which an engine is mounted on a front portion of a vehicle body frame, a driver's part is provided at a position behind the engine, a transmission casing is provided at a position below a driver's seat mounted in the driver's part, a PTO transmission mechanism is mounted in the inside of the transmission casing, and a PTO shaft which is interlockingly connected with the PTO transmission mechanism projects rearwardly from a rear wall of the transmission casing (see Japanese Patent Laid-open Hei7(1995)-232570, for example).
Further, as a type of tractor, there has been known a tractor in which a proximal end portion of a lift arm is pivotally mounted on an upper portion of the transmission casing, a lift cylinder mounting portion is provided in a state that the lift cylinder mounting portion projects rearwardly from a rear wall of the transmission casing, and a lift cylinder which extends or contracts in the vertical direction is interposed between the lift cylinder mounting portion and a middle portion of the lift arm (see Japanese Patent Laid-open Hei9(1997)-109710, for example).
However, in the above-mentioned tractor, since the PTO transmission mechanism is mounted in the inside of the transmission casing, it is necessary to perform the assembling operation and the maintenance operation of the PTO transmission mechanism in the inside of the transmission casing whereby there arises a drawback that the operation becomes cumbersome and, at the same time, there also arises a drawback that the transmission casing per se is large-sized in the rearward direction.
Further, in the above-mentioned latter tractor, since the lift cylinder mounting portion is provided in a state that lift cylinder mounting portion projects rearwardly from the rear wall of the transmission casing, the adjustment to arrange the lift cylinder which is interposed between the lift cylinder mounting portion and a middle portion of the lift arm vertically or substantially vertically becomes cumbersome.
That is, since the transmission casing becomes large-sized in the rearward direction, when the lift cylinder is simply interposed between the lift cylinder mounting portion which is provided in a state that the lift cylinder mounting portion projects in the rearward direction from the rear wall and the middle portion of the lift arm, the lift cylinder assumes an inclined posture. Eventually, there arise drawbacks that a large lift cylinder having a long stroke becomes necessary and, at the same time, a power loss is generated.
Accordingly, by pivotally mounting a proximal end portion of the lift arm as close as possible to a rear end portion of the transmission casing or by elongating an arm length of the lift arm, the posture of the lift cylinder may be arranged vertically or substantially vertically. However, the arrangement requires a cumbersome operation that the arrangement must be performed only after confirming the change of strength of the pivotal mounting structure of the proximal end portion of the lift arm, the kinetic change (the change of the moment) brought about by the change of the arm length of the lift arm, change of the elevation and lowering locus of a working machine which is connected to the lift arm and the like.